As Mojo said, you can talk to potential clients about projects and show them work you’ve done in the past. Based on these preliminary discussions, you and the potential client would decide whether or not you want to work together.
What you should not do is start working on any ideas until the client has signed a contract and (depending on the situation) given you a deposit that ensures you’ll be paid for any initial work that you do.
In other words, preliminary discussions are typically needed before deciding to move ahead with a contract No actual work is done or shown to the client until after the contract is signed. When clients want to see ideas before agreeing to hire you, you tell them “No”.
There are sometimes exceptions to this when you’ve identified a client you want and you’re taking the initiative of pitching an idea to them. In that situation, you’ve approached the potential client and you’re trying to convince them to hire you by showing them a great idea you’ve worked up. Even doing this is controversial, however, and many designers refuse to make these kinds of pitches.
This is all very different from crowdsourcing website work, which often involves designers doing work for potential buyers with no assurance of getting paid unless the buyer decides to purchase the work that’s been developed. This is the main reason (among others) so many of us dislike crowdsourcing.